From the resulting set, we reduce to the values of the attributes..map(&:value)

Then, we select only values starting with “http”.select {|e| e[0..3]=='http'}

Finally, we remove any URLs pointing to resources on the same domain..select {|e| e.match(URI.parse(url).host).nil?}

We’re left with an array of URLs linking to external resources. If any of these links are dead, the user experience will suffer. Uptimetry (http://uptimetry.com) already offers a powerful cloud-based solution for URL monitoring. Soon, we will offer the option to crawl your web properties automatically for external links to monitor, saving you time and giving you peace of mind every month.

Like this:

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The parsing of the URL in the last phase of the processing just to get the host could be fairly expensive on pages with many hrefs. Also e[0..3] generates additional objects (which you might be sensitive to). #select plus a nil? check can typically be expressed with reject instead (though one might argue it is less readable) — Also the #match could be include?

I might suggest something like the following instead (though the pre-extraction of the host value kinda kills your ‘one liner’ bit)